Polydienes are most often produced by solution polymerization, wherein conjugated diene monomer is polymerized in an inert solvent or diluent. The solvent serves to solubilize the reactants and product, to act as a carrier for the reactants and product, to aid in the transfer of the heat of polymerization, and to help in moderating the polymerization rate. The solvent also allows easier stirring and transferring of the polymerization mixture (also called cement), since the viscosity of the cement is decreased by the presence of the solvent.
Nevertheless, the presence of the solvent presents a number of difficulties. The solvent must be separated from the polymer and then recycled for reuse or otherwise disposed of as waste. The cost of recovering and recycling the solvent adds greatly to the cost of the polymer being produced, and there is always the risk that the recycled solvent after purification may still retain some impurities that will poison the polymerization catalyst. In addition, some solvents such as aromatic hydrocarbons can raise environmental concerns. Further, the purity of the polymer product may be affected if there are difficulties in removing the solvent.
Polydienes may also be produced by bulk polymerization (also called mass polymerization), wherein the polymerization mixture is typically solventless; i.e., the monomer is polymerized in the absence or substantial absence of any solvent, and in effect, the monomer itself acts as a diluent. Since bulk polymerization involves mainly monomer and catalyst, there is reduced potential for contamination and the product separation may be simplified. Economic advantages including lower capital cost for new plant capacity, lower energy cost to operate, and fewer people to operate may be realized. The solventless feature may also provide environmental advantages with reduced emissions and wastewater pollution.
Nonetheless, bulk polymerization may require careful temperature control, and there may be a need for strong and elaborate stirring equipment since the viscosity of the polymerization mixture may become very high. In the absence of added diluent, the cement viscosity and exotherm effects may make temperature control very difficult. Also, cis-1,4-polybutadiene is insoluble in 1,3-butadiene monomer at elevated temperatures. Consequently, local hot spots may occur, resulting in degradation, gelation, and/or discoloration of the polymer product. In the extreme case, disastrous “runaway” reactions may occur.
Olefins, which are distinct from conjugated dienes, have commonly been polymerized by gas-phase polymerization or slurry-phase polymerization techniques that employ solid-supported catalysts. These gas-phase or slurry polymerization processes have been plagued by reactor fouling or sheeting, which has caused operability problems. For example, fouling of gas-phase polymerization reactors during the production of polyethylene or polypropylene is a well known problem. It is believed that this fouling is caused by an uncontrolled reaction caused by catalyst embedded within polymer stuck to reactor or pipe surfaces.
The prior art has addressed the problems of fouling or sheeting within gas-phase or slurry-phase reactors employed for olefin polymerization by employing several approaches. For example, U.S. Pat. No. 6,632,769 discloses the use of additives that change phase when heated and thereby release a catalyst poison. U.S. Pat. No. 6,346,584 describes the use of a binary system that reacts above a desired threshold temperature to generate a catalyst poison. U.S. Pat. No. 6,713,573 discloses the use of additive systems that undergo thermal decomposition at temperatures above a desired threshold to generate a catalyst poison. U.S. Pat. No. 4,942,147 discloses a transition metal catalyst system containing an autoacceleration inhibitor.
Since the advantages associated with bulk polymerization systems are very attractive, there is a need to improve bulk polymerization systems. Further, a method is needed to prevent runaway reactions in liquid-phase bulk polymerizations.
Arriving at a solution to prevent runaway reactions during lanthanide-catalyzed conjugated diene polymerization, however, is not trivial. Unlike gas-phase or slurry-phase olefin polymerization, the bulk polymerization of conjugated dienes occurs in the liquid phase. And, the catalyst system is dissolved in the monomer/polydiene mixture. Moreover, lanthanide-based catalyst systems are notoriously susceptible to impurities. That is, various impurities can have a deleterious impact on these catalyst systems and the polymerizations in which they are used.